Project Abstract Extreme heat is a major public health problem that has caused more deaths than other weather related causes in the U.S. Farmworkers are vulnerable to extreme heat because nearly 85% of U.S. farmworkers labor outdoors and they are usually engaged in heavy work, e.g., the rate of heat-related deaths in the agriculture/forestry/fishing and hunting industry was the highest among all industry sectors. Projected increases in the frequency and intensity of heat waves together with an aging workforce are expected to worsen vulnerabilities of farmworkers to heat stress in a changing climate. Migrant and seasonal farmworkers are likely more vulnerable to heat than other workers because poverty, limited health insurance coverage and unauthorized work status might exacerbate their vulnerability. However, to our knowledge, no studies have quantified the associations between extreme heat and heat-related illness among migrant and seasonal farmworkers in the U.S. using medical records and evaluated potential vulnerability indicators. Our long-term goal is to reduce heat-related health effects for migrant and seasonal farmworkers in the U.S. We are uniquely poised to achieve this goal because we can leverage resources from the first longitudinal medical records database (Community Based Research Network [CBRN]) among migrant and seasonal farmworkers in the U.S. Specifically, we propose: 1) to determine whether excess cases of all-cause and cause-specific (cardiovascular, respiratory, diabetes, renal and heat-related causes) illness occur among migrant and seasonal farmworkers included in CBRN on hot days; and 2) to evaluate potential determinants of vulnerability by assessing whether heat-morbidity associations differ, according to farmworker status, individual characteristics, and geographic areas. This project addresses a significant, but rarely studied extreme heat- related public health problem in a vulnerable population, and has implications for many other outdoor workers. This highly time- and cost-effective study will be the first to investigate heat-related illness on migrant and seasonal farmworkers using medical records in the U.S. The results of this study can help support evidence- based policy development, develop and evaluate targeted interventions, and improve surveillance of occupational injuries and illnesses that is greatly needed in the agriculture sector. The work is consistent with priorities identified by the National Occupational Research Agenda related to the Agriculture sector Intermediate Goal 5.4. This research contributes to the NIOSH research-to-practice (r2p) initiative by disseminating the research results to key stakeholders.